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Tiny insects pose huge risk for black walnut trees

Thousand cankers disease (TCD) is not yet in Michigan but has already been discovered in Ohio and Indiana. This fungal pathogen is responsible for spreading TCD, threatening the widespread health of black walnut trees across Michigan.

The walnut twig beetle’s tiny size make them very hard to detect on trees. Photo by: Whitney Cranshaw, Colorado State University

Everybody roots for the little guy, right? Rooting for the underdog always seems to capture both sports fans’ and the media’s attention. In the case of a new invasive forest health problem threatening black walnut (Juglans nigra) trees, it’s a “little guy” that poses a huge threat to our black walnut resource in much of the United States.

The invasive pest problem that is threatening black walnut trees is called thousand cankers disease (TCD). It is described as a disease or pest “complex” because it requires both a tiny beetle as well as a fungal pathogen to invade and kill black walnuts. The Walnut twig beetle (Pityophthorus juglandis) is the insect most responsible for spreading TCD to uninfected trees. The adult beetle is only about 1/8 of an inch in length. Its extremely small size makes it very difficult to spot, since the twig beetles usually attack the branches in the top of trees first.

The walnut twig beetle can carry spores of the pathogen Geosmithia morbida, (obtained from boring into other infected walnuts) and spreads them to healthy walnuts as it tunnels underneath the bark of twigs and small branches. The result is the formation of tiny dead (necrotic) areas or cankers around the area that was bored by the beetle. Because many beetles can attack a walnut tree at the same time, the result is a multitude of cankers being formed – hence the name thousand cankers disease. Given enough time and infestation, a black walnut will eventually die from repeated infections of TCD.

Multiple small cankers end up girdling branches, eventually killing the tree. Photo by: Whitney Cranshaw, Colorado State University

Fortunately, TCD has not yet been found in Michigan. But the fungal pathogen that causes the canker or the twig beetle that spreads it have been found in both Ohio and Indiana – just south of the Michigan state line. So, the threat posed to Michigan from TCD is indeed very real!

According to the Michigan Deparment of Agriculture and Rural Development, Michigan’s forests contain over 8.5 million black walnut trees with an economic value of more the $86 million. Black walnuts also possess a high ecological value as a food source for birds, mammals and other wildlife. In addition, there are more than 80 walnut growers in Michigan with approximately 4,000 trees in nut production.

Michigan residents who are interested can help keep TCD out of Michigan by becoming Sentinel Tree volunteers with the Michigan Eyes on the Forest and Sentinel Tree Network. Funded by the Michigan Invasive Species Grant Program, the MSU “Eyes on the Forest: Invasive Forest Pest Risk Assessment, Communication and Outreach Project,” links research with outreach and communication projects through the MSU Department of Entomology and Michigan State University Extension.

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